A recent retrospective data analysis from the laboratory of Dr. John Barrett of NHLBI studied the relationship between peripheral blood counts post transplant, chimerism, and clinical outcome in patients undergoing allotransplantation for the treatment of recurrent leukemia. The studies demonstrated that a high concentration of circulating lymphocytes at day 30 posttransplant was a positive predictor of rapid donor engraftment in the myeloid compartment, reduced recurrence of AML, reduced GVH, and improved survival. Additional analysis indicated the level of circulating NK cells at day 30 to be a particularly important predictor of outcome. These studies also demonstrated that the infusion of a larger CD34 cells dose at the time of transplantation also favorably affects these measures of clinical outcome.[unreadable] [unreadable] Several recent protocols treat stem cell recipients with stem cells from two or more donors to hasten reconstitution or treat leukemic recurrence. In some cases, this involved the infusion of human umbilical cord stem cells from two separate donors. In other cases, a combination of peripheral blood T cells and stems cells from an HLA-matched donor plus CD34 cells from another haploidentical relative were used in combination. Individualizing a strategy to unequivocally identify cell populations from 2 or more related donors and compute % chimerism has been technically challenging, but to date we have been able to successfully distinguish cells from the recipient and each of the donors in every case. We anticipate the continued attempts to use multiple donors for engraftment in the coming year. [unreadable] [unreadable] A recently adopted NCI protocol uses highly T cell depleted stem cell preparations to reconstitute patients with aggressive lymphoproliferative malignancy. In several instances, disease progress in this group has forced institute investigators to administer additional chemotherapy shortly after donor cell transfer. Consequently, we have noted early graft failure in several of these patients. Through close monitoring of chimerism for graft failure and frequent communication with the responsible investigators, these patients were promptly recognized and retreated with a second stem cell infusion. In each case, successful engraftment was obtained after the second infusion. The findings illustrate the extreme sensitivity of recently transplanted stem cells to chemotherapeutic agents.